Twenty years after it was placed in Santa Monica's Civic Center, the anti-nuclear war symbol is fenced off to keep the public away.

Cartoonist Paul Conrad would probably draw pleasure from the newest fallout surrounding his anti-nuclear war sculpture in Santa Monica.

Two decades after its controversial placement in their downtown Civic Center, city officials worry that the stylistic mushroom-cloud artwork depicting the horror of atomic warfare is falling apart.

Exposure to salt air has caused the sculpture's fiberglass base to deteriorate, loosening some of the fasteners that hold the intertwined chains that form the 26-foot mushroom cloud.

Workers erected a temporary fence Monday around the 51/2-ton creation, which Conrad called "Chain Reaction."

"We've had kids climbing it, teenagers climbing up to touch the top," said City Manager Rod Gould, who was on his way to lunch Tuesday when he paused at the base of the piece. The sculpture sits on a concrete pad next to Main Street, between the courthouse and Civic Auditorium. Steel framework hidden inside the fiberglass substructure supports it. The artwork's chain is made of copper tubing.

"We'll bring in an independent structural engineer and conservators to determine its soundness," Gould said. "You really can't determine it just by looking at it from the outside. We're alerting Paul Conrad's heirs that we have this problem."

City Building Officer Ron Takiguchi first sounded the alarm about the sculpture's condition after noticing children climbing on it, said Jessica Cusick, Santa Monica's cultural affairs manager.

Conrad, a longtime editorial cartoonist for The Times, died in September. Along with the newspaper work that earned him three Pulitzer Prizes, he was a skilled artist who produced a series of whimsical bronze sculptures depicting American political leaders.

"Chain Reaction" was placed in Santa Monica in 1991, four years after Conrad offered it to the city. From the start, however, it was controversial.

A scale model of the sculpture was displayed at City Hall next to a ballot box, and visitors there voted against it, 730 to 392. Nonetheless, the Santa Monica Arts Commission unanimously voted four times to accept the piece, and the City Council finally agreed, voting 4 to 3 in favor of it in 1990.

Those puzzling over the sculpture's new fence Tuesday said they hope that any repairs are done quickly.

"I've never seen a kid climb on it and I walk by here every day," said Chris Benson, who operates a Santa Monica software company. "This is something the city has got to keep."

Salesman Stewart Howe, a Santa Monica native, wondered if pressure to remove the mushroom cloud might be coming from the Rand Corp., the think tank best known for its governmental security analysis work. Rand moved to a new headquarters directly across the street from the sculpture in 2004.

"This sculpture is telling us what Rand actually does," he said.

Rand Corp. spokesman Warren Robak denied that officials have complained about the antiwar artwork. "It's part of the neighborhood," Robak said of the sculpture.

Longtime Santa Monica activist and peace advocate Jerry Rubin predicted protests if the city attempts to remove the artwork.

"I've never seen anyone climb on it. But you can remedy that by putting up 'keep off' signs," said Rubin, one of those who lobbied for the acceptance of "Chain Reaction" in the late 1980s.

He said Conrad enjoyed riling up people with his pointed cartoons and his controversial sculpture.

"This is important artwork about an important issue. If there's corrosion, get a welder and fix it. It's going to fall over? Please."